The father tells his son he must go on in order to "carry the fire." He says: "It's inside you. It was always there." [p. 279]. What is this fire?

His father is referring to the human spirit, the will to
survive that has kept them going from the moment of the disaster, through the suicide of
his wife and the son's mother, and through every horror and tragedy they have
experienced on the long road to the coast.

In essence, this
is why the book, even though it seems thoroughly depressing most of the time, is
actually a story of hope and survival.  Humans will do almost anything to survive, and
these two characters have, obviously, at great personal expense to their psyches and
their physical well being.  The struggle takes the father's life, and in this quote, he
is telling the son to survive and carry on.